<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>margin 负值</title>
</head>
<style>
    ul li {
        float: left;
        list-style: none;
        width: 150px;
        height: 200px;
        border: 1px solid red;
        margin-left: -1px;
    }

    ul li:hover {
        border: 1px solid skyblue;
    }
</style>
<body>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
    </ul>
</body>
</html>